He was signed by the Kansas Cty A’s in February 1963. He was sent to the Daytona Beach Islanders, in the Florida State League. He was more of a swingman, starting nine games in 36 total appearances. It would be his only year when he would be used significantly as a starting pitcher. His ERA was 4,15. His control was off a bit, allowing 78 walks in 115 innings, but he was hard to hit: in 115 innings, he allowed 100 hits, the best ratio on the team.

Lachemann played in Lewiston in 1964, in the Northwest League. Among regular pitchers, his 2.81 ERA in 48 innings was the team’s best. He cut down on his walk total (18). In 26 games (one start), his record was 3-4. He moved up to AA Birmingham in 1965, in the Southern League. He led the team with 58 appearances (second in the league to Andrew Dustal) but also in losses (11). His ERA was 3.32 in 95 innings. He kept moving up the ladder in 1966 with the Vancouver Mounties, in the Pacific Coast League. Again, he was his team’s most-used reliever, with 53 games. In 97 innings, his ERA was 4,18, allowing 89 hits and 53 walks. He tied Bobby Tiefenauer for 5th in the 1966 PCL in games pitched.

In 1967, he was back to AA in Birmingham but struggled. In 35 games, his ERA was 4.57 in a pitcher-friendly ERA (3.02 league ERA). In 63 innings, he allowed 62 hits and 29 walks. He began 1968 with Birmingham, where he completely dominated. In 22 games covering 45 innings, his ERA was 0.80. He was promoted to Vancouver, where his ERA was 2.73 in 14 games (33 innings).

Des Moines, Iowa was the A’s new affiliate in 1969 (as the Iowa Oaks) and that’s where Lachemann began that season. He was the team’s best reliever. In 20 games covering 51 innings, his ERA was 2.47. His five saves remained a team best at the end of the season. In June, after six and half years in the minors, he was called up in June by the Oakland A’s. In his first game, he allowed two runs to Baltimore. Then, he pitched nine straight games when he didn’t allow an earned run, lowering his ERA at 1.15. During that stretch, he won his first two decisions and would have been credited with two unofficial “holds” (the statistic did not yet exist). On August 6, his ERA was still a solid 2.05. After allowing two runs two days later, he was sent down to the minors. He was back in Oakland a month later. One very bad outing in mid-September hurt his final stats (five earned runs in two thirds of an inning). His final record with Oakland was 4-1 in 28 games with a 3.95 ERA. From 1969-1972, his brother Rene would be his teammate with Iowa.

Lachemann began 1970 with Iowa as the team’s #1 reliever. After 11 games, over which he was involved in nine decisions (3-2 record with four saves) with a 2.45 ERA, Lachemann was called up by the A’s in May. He became part of a very solid group of relievers, led by Jim "Mudcat" Grant, along with Paul Lindblad and Bob Locker (the whole relief group would show a 2.55 ERA). As for Lachemann, he pitched in 41 games, splitting his six decisions to go with three saves, with a nice 2.78 ERA.

He began 1971 with Oakland at last, but Lachemann couldn’t enjoy the moment very long. He pitched only one game before being sent down to AAA, never to be back in the big leagues again (as a player). He was Iowa’s closer in 1971, leading the team in games (43) and saves (11), while showing a team-best 3.04 ERA. He was 9th in the 1971 American Association in pitching appearances and tied Dennis Higgins and Aurelio Monteagudo for second in saves, 7 back of Garland Shifflett.

It was back to Iowa in 1972. For the first time since 1967, he struggled, allowing 81 hits in 72 innings that led to a 4.88 ERA (6-7, 5 Sv). The A’s released him in January 1973. In February 1973, the Montreal Expos organization signed him to be a player-coach with their AA affiliate in Quebec, in the Eastern League. It was the beginning of a way longer career in the coaching ranks, a path he had envisioned before he even reached the majors. His main duties were as a coach, but he was still asked to pitch in 25 games, including one start. He was hit harder than ever, allowing 61 hits in 40 innings for a 4.28 ERA. He went 2-3 with two saves.

Lachemann remained in the organization two more seasons as a pitching coach in 1974 and 1975 with West Palm Beach. It looked like a move down but Lachemann had asked himself to be sent elsewhere as he found the setup in Quebec hard for the whole family. He saw some action in West Palm Beach in 1974, pitching five games in relief. After 1975, farm director Mel Didier left the Expos and his successor, Kevin McHale, asked him to go back to Quebec City. Lachemann declined and then joined his alma matter USC as an assistant coach in 1976, a position he would hold until 1981. In the meantime, he also managed in the collegiate Alaska Baseball League in 1976-77, leading the Anchorage Glacier Pilots to the circuit championship.

In 1982, he joined the California Angels organization as a roving pitching coach for two seasons. He then became the Angels’ pitching coach from 1984 to 1992. In 1993, he became the Florida Marlins' first-ever pitching coach, under the managership of his brother, Rene Lachemann. He would remain in that position until May 1994, leaving the Marlins to become the Angels manager. In 1995, he led them to a tie with the Seattle Mariners for the top spot in the AL West, but lost the tie-breaker due to the pitching of Randy Johnson. He resigned in May 1996 but remained in the coaching staff as the pitching coach until 1998. After a year off, he was back as a pitching coach, this time with the Colorado Rockies, a position he held for two seasons in 2000 and 2001. He remained in the organization until the end of 2011, at what time he had become a special assistant to the GM. After the 2011 season, he resigned, citing differences in philosophies. Within weeks, he was hired by the Los Angeles Angels as a special assistant to GM Jerry DiPoto.